(a) Field of the Invention
The present invention relates to a stereophonic signal processing circuit producing, from 2-channel stereophonic signals, a center channel signal to be located midway between them.
(b) Description of the Prior Art
In the past, attempts have been made that a loudspeaker for a center channel is arranged midway between individual loudspeakers for a 2-channel stereophonic sound system, or alternatively a loudspeaker for a center channel is arranged midway between individual loudspeakers for two channels provided in the front, among individual loudspeakers for a 4-channel stereophonic sound system, thereby improving the localization of a sound field. FIG. 1 shows a general arrangement as described above.
Referring now to FIG. 1, stereophonic signals L, R supplied to a pair of stereophonic signal input terminals 1L, 1R are reproduced as sounds by loudspeakers 2L, 2R, respectively through power amplification. A center channel producing circuit 3 is adapted to produce a center channel signal C to be reproduced by a loudspeaker 2C for a center channel in accordance with the stereophonic signals L, R. Further, a reproduced sound derived from these signals is picked up by the ears of an audience 4 with the result that sound lateralization is brought about.
For a producing system of the center channel signal, various ones have been known from the past. There is the system, as a typical one, that a simple monaural output is supplied to the loudspeaker 2C for the center channel. This is such that, for example, the signal of an L channel is added to that of an R channel and then these signals are multiplied by 1/2 to produce the center channel signal.
Also, a system referred to as Dolby Surround "Pro-logic" proposed by Dolby Laboratories (Dolby is a trademark of Dolby Laboratories Licensing Corp.) has outputs for a total of four channels provided on the front and rear sides and the left and right sides with respect to stereophonic signal inputs comprising an L channel and an R channel. Further, the outputs on the left, right, front (center) and rear sides are subject to control by a VCA (voltage control amplifier) in accordance with individual output levels of L, R, L+R and L-R so that the sound lateralization of the reproduced sound is caused.
In the foregoing prior art, however, the following problems have been encountered.
First, in the case of the simple monaural output, the sound lateralization of the reproduced sound cannot completely be brought about. Specifically, since the center channel output C is determined from EQU C=(L+R)/2,
if L=R, the center channel output C attains the same level as in L and R channel outputs, while on the other hand, if L=0 for example, the center channel output C is reduced to the level of half of the R channel output. Accordingly, even if the output of the R channel only is obtained, the output of half (a quarter in sound pressure) of the R channel output is generated from the center channel and as such the localization of the reproduced sound is under a bias toward the inside (L side).
Second, in the case of Dolby Surround "Pro-logic", not only the complication of the system causes level setting to be difficult, but the L and R channel signals themselves are processed, with the result that their qualities are deteriorated.